1. Field of the Invention:
This invention relates to an information signal transmission system and more particularly to an information signal transmission system for transmitting a code sequence by predictive differential coding in which, mutually adjoining codes are correlated to each other.
2. Description of the Prior Art:
Generally, a digital signal obtained by sampling and then quantizing an analog signal such as an audio signal, or a video signal having a time-wise correlation, and by further subjecting them to analog-to-digital conversion, that is, a code sequence consisting of, for example, pulse-code-modulated (PCM) code has the mutually adjoining codes thereof correlated to each other. Heretofore, in transmitting the code sequence of this kind of a transmission system and recording or reproducing it on or from a record bearing medium, a code frame has been formed with at least one code group consisting of a plurality of information codes, a synchronizing codes and an error detecting-correcting code and the transmission has been carried out frame by frame.
FIG. 1 of the accompanying drawings shows an example of the conventional code frame to be transmitted as mentioned above. Referring to FIG. 1, a leading code 1 is a synchronizing code. A reference numeral 2 denotes information code and a numeral 3 an error detecting-correcting code. The synchronizing code 1 and the error detecting-correcting code 3 represent redundant code required for transmission of digital data. For example, with the number of bits of the error detecting-correcting code 3 increased, an error detecting and correcting capability of a code receiving arrangement can be enhanced against an error occurring in the transmission line.
However, the redundant code or the increase in the number of bits thereof, results in the increase of transmission signal frequency in the transmission of codes and that of the transmission band. Then, the transmission tends to be affected by the frequency characteristic of the transmission line and external noises. This eventually causes an increase in transmission errors. Further, in the case of recording or reproduction on or from a record bearing medium such as a magnetic tape, the arrangement eventually shortens the recording wavelength. Then, the transmission would be readily affected by the frequency characteristic of the record bearing medium, dust, scars, signal drop-outs, etc. Further, in cases where the number of quantization bits of each information code is increased for improved quality of a received or reproduced analog signal, this likewise results in an increase in the transmission frequency and in shorter recording wave length. Then, that arrangement also presents the same problem.
To moderate the increase in the rate of such code, there has been employed a high efficiency encoding technique, which includes varied methods. Of these methods, it has been most popular to effect code conversion by a differential PCM (hereinafter will be called DPCM) method. In the DPCM method, a next digital code is predicted from a past information code and only a difference between a predicted valve and a current code (predicted error) is quantized by a certain quantization step. Compared with the above-stated ordinary information signal transmission arrangement, the code transmission method which performs the code conversion by the DPCM method permits reduction in the number of bits required for transmission in cases where signals of the same quality are to be transmitted.
Further, code conversion by an adaptive differential PCM (hereinafter will be called ADPCM) method has recently been proposed for further reduction in the code rate. The ADPCM method moreover permits the quantization step to be variable. According to this method, the quantization step width .DELTA. is determined as a functional value of the past codes. With this functional value suitably selected, code transmission can be more efficiently carried out than by the DPCM.
However, in transmitting codes in accordance with such a high efficiency encoding technique, the predicted value must be restored from the past code at the time of decoding. Generally, for the prediction, one code of the past becomes the predicted value, as in the case of a one-dimensional pre-value prediction method, or a plurality of past codes as in the case of a linear prediction method are used. In either of these methods, once an error arises in a transmission line, a next code decoded with a predicted value which is restored by using this erroneous code also becomes erroneous. Then at the decoding process, the error endlessly propagates. Further, with the above-stated ADPCM method employed, the quantization step width .DELTA., which is obtained as the functional value of the past codes must be also restored at the time of decoding. Accordingly, in the event of occurrence of an error in the transmission line, a next code which is decoded by the erroneous quantization step width also becomes erroneous and thus the error also endlessly propagates.